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Lake Powell approaching historic low levels

Posted at 6:20 PM, May 18, 2021
and last updated 2021-05-18 21:38:56-04

LAKE POWELL, Utah — The Colorado River system — and the tens of millions of people relying on it in America’s southwest — faces a crisis of drought and consumption that is most easily seen by looking at the condition of its reservoirs.

In southeastern Utah, Lake Powell is a shadow of itself. In the chart below, we looked at how much water the lake has held for the last five years.

Some things to note:

  1. Capacity is 27 million acre-feet, so you see the lake has been consistently low throughout this time.
  2. The shaded areas show how spring runoff has replenished the water supply each year, except 2021 where forecasts from the Bureau of Reclamation now estimate inflow from the Colorado, San Juan and smaller tributaries will not exceed outflow.

The question we asked: Is the current downturn normal for the Lake’s cycle? To answer, we examined every spring since 1963 and charted all since 1970.

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Here’s a look at all those charts. Some things to note:

  1. These charts show cubic feet per second (cfs) flowing in. It’s the standard measure for stream flows tracked for over a century, but simply indicates the amount of water that flows either by a point or into a point at one moment in time. In this case, it’s averaged out over each month.
  2. The Lake does go through cycles. Each decade has seen down years, though this year is extraordinary.
  3. Each decade’s chart has a different scale to match the highest average cfs within the time frame, so the 1980s chart extends above 80,000 cfs.

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After looking through those, and the inflows in the 60s as well, we compared Spring 2021 thus far to the 11 other worst years for spring runoff:

Seeing those inflows, this last chart should be no surprise. These are the four months of the lowest average lake levels on record since 1970, after the reservoir was initially filled.

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Note for this chart:

  1. The previous lowest monthly averages were January-March of 2005. We’d expect those to be the lowest months as the lake replenishes in spring with levels ebbing until the next spring. Seeing April of this year as next on the list bucks that trend and shows the significance of the drought.
  2. The final number is the current “most probable” projection for the lake level at the end of 2021. In other words, in the coming months, Lake Powell will see new record low elevations every month until the west exits its drought.
  3. The numbers are feet LOWER than capacity, so a higher number means a lower lake.